Update a part of a string in code (Access 2003, XP

I don't know if this is possible, but it'd be great if it was...

I am running a project that relies on a fixed-width data string and can parse out various fields in code for examination using Mid(FieldNameDesired, startpoint, len). now I want to reverse the operation and write back to a copy of the string any changes to _just that section of the string_.

This SQL doesn't work, and I'm not terribly surprised:

UPDATE EditMDS_String SET MID(MDSRecord_Edit, 260, 2) = '01';

Any ideas? I am trying to avoid re-writing the entire datastring as, 1) the fields we are extracting and operating on are a small % of the overall string and 2) any errors at all may be hard to detect or verify.